First published in Pulp, Paper and Logistics

Digitalization should be a part of every company’s strategy to move the pulp and paper industry towards autonomous operations, driving productivity and profitability improvements. Advanced Process Control (APC) is increasingly being used by mills as an enabler in their digital transformation. It serves as a link between low-level automation systems and higher-level management such as production planning systems.

APC has become indispensable to many operators today. The implementation of flexible APC solutions allows for the integration of economic optimization strategies to help lower production costs and greater adaptation to changes in targets or seasonal conditions, for example. Many more developments are likely to follow in the next few years to help mills stay competitive.

APC solutions today are already proving to be versatile with an extended functionality that can adapt not only to a target but to a range as well, allowing the best operating regime to be found. This ultimately leads to significant savings without compromising on quality.

One of the fundamental tools in any APC is model predictive control (MPC), which is a set of algorithms for feedback and feed-forward control based on a process model. While MPC has been around since the 1980s, it is relatively new for the pulp and paper industry -  in large part due to the complexities of the process. Kraft pulping, for example, has many interacting processes, varying dynamics, and conflicting objectives, and is extremely difficult to model and control. It can also be easily disturbed by many factors such as raw material quality, equipment age and the ambient weather. 

Over the years, ABB has developed additional tools to further optimize process automation and make APC into a more robust and adaptable form, leading to easier and more successful adoption for pulp and paper processes. Areas where ABB APC solutions have been successfully implemented include pulp digesters, brown stock washers, bleach plants, lime kilns, causticizers, and multi-effect evaporators as well as paper machines. 

1. Having accurate sensors, robust actuators, and a reliable distributed control system (DCS) in place is critical. These are the foundation on which the APC is delivered and if the measurement system or DCS is not up to scratch, the APC will not be able to deliver the expected benefits.
2. Ability to combine operational data with process knowledge is paramount. Ensuring that the APC is not a black box but can incorporate the operators experience into its models along with the data-based models is key to achieving the targets with minimal efforts.
3. Transparency and ease of use is critical to get the operators’ buy-in. It is very important that the operators can relate with the decisions that the APC makes, visualize the predictions, and use the system intuitively for fast and efficient start-ups.

Once APC is implemented, it is equally important to continuously monitor performance and share key performance indicators so it can be sustained. ABB believes that performance can be sustained, and even improved, over many years through close collaboration with a service-based model for long-term successes. 

Following the evolution of these critical factors has resulted in  APC being applied to many different processes. Take for example the lime kiln operation, which is one of the major consumers of energy in the kraft process and exhibits a lot of variability in energy use, quality and productivity. Although the lime kiln process is straightforward in principle, kiln operators still face difficult issues such as low thermal efficiency, high fuel consumption, process delays, build-up of rings and dust, overheating of refractory, poor quality of lime and increased emissions. OPT800 Lime manages these challenges to achieve optimum control and efficient lime production. 

The solution works to optimize lime production rates, reduce energy consumption and emissions, increase re-burned lime availability, decrease residual carbonate variations, and improve overall operation with increased visibility of the process. It combines laboratory results by operators with MPC, thus getting the best of both approaches in a fully product-specific solution, with minimum need for on-site development. This enables tighter control of the lime production process, leading to more stable pulp mill operations.

Looking to the fiberline side of pulp mill operations, mills can achieve optimum pulp quality leveraging APC in the cooking process. When ABB’s OPT800 Cook/C APC module is applied in a continuous pulp digester, it controls an estimated Kappa number in the cooking zone, residual alkali at the extraction stage, chip level in the digester and blow consistency. Typical results include reduced variability in the final Kappa number and reduced steam and alkali consumption, resulting in lower energy and bleaching costs.

Another example of APC implementation is on the wet end of a paper machine. Producing paper on a paper machine involves a series of highly complex processes and variation in any of these processes could lead to the formation of a defect in the sheet or a weakness in the paper web, eventually leading to a sheet break or rejection due to poor quality. Wet End Control helps to monitor and analyze the wet-end processes and can significantly reduce downtime, the usage of chemical additives and production costs, while maximizing production.